Advances in high density magnetic storage systems depend largely on improvements in the magnetic data storage media. The properties of the media are determined by the composition and the structure of the active magnetic storage material and contiguous layers within the media structure. For high density longitudinal recording media the important characteristics are coercivity, signal-to-noise ratio, remanent magnetization, coercivity squareness, layer thickness and surface roughness.
It is known, for longitudinal magnetic recording with a ring head that the linear recording density (LARD) of the magnetic media is governed by the following relationship: EQU LRD .varies.1/a
where a is a parameter given approximately by the relation: ##EQU1##
where M.sub.r is the remnant magnetization, t is the thickness of the magnetic layer, d is the distance from the pole tip bottom of the magnetic write or read device to the top of the magnetic layer and H.sub.c is the coercivity. Furthermore, it has also been established that the output is related to M.sub.r and t as follows: EQU OUTPUT.varies.M.sub.r.multidot.t
where M.sub.r.multidot.t is called the magnetization thickness product. Hence, in designing a magnetic data storage media having a high recording density, it is necessary to increase H.sub.c while maintaining a low M.sub.r t value that is still sufficient for the required output application. According to prior art, to support longitudinal magnetic recording at densities close to 10 Gbit/inch.sup.2 the coercivity H.sub.c of the magnetic media must range between 2,500-4,500 Oe and the material grain size should be approximately 100 Angstroms; Glijer et al., "Structural Design of CoCrPt(Ta,B)/Cr Magnetic Thin Film Media for Ultra High Density Longitudinal Magnetic Recording", Scripta Metallurgica et Materialia, Vol. 33, Nos. 10/11, 1995, pp. 1575-84.
Small grain sizes are important for the next generation magnetic data storage media because small grain sizes can significantly increase the media signal to noise ratio (SNR). The media SNR is a ratio of the output signal over media noise, and can be described by the equation; EQU SNR.varies.WB/D.sup.2
Here W is the track width, B is the bit size and D is the in-plane grain diameter of the active magnetic layer. Hence, the SNR of the magnetic media varies inversely as the square of the in-plane grain diameter and a significant reduction in the magnetic media noise can be achieved by reducing the grain size.
Traditionally, making magnetic data storage media involves coating magnetic particles in a resin or binder. Recent efforts to achieve high density magnetic media have focused on thin layer alloy magnetic structures. The thin layer alloy magnetic media are made by several methods including sputtering, vapor deposition, ion-beam deposition and electroplating.
Thus far, the most successful thin layer magnetic alloy materials for data storage are based on cobalt-chromium alloys with additions of platinum, tantalum, boron and alloys thereof. Examples of such magnetic media are described by Murayama et al. in U.S. Pat. No. 5,478,661, by Oka et al. in U.S. Pat. No. 5,494,722 and by N. Tani et al. in "High Coercivity Hard Disk with CoCrPtB/Cr Media", IEEE Transactions on Magnetics, Vol. 27, No. 6, November 1991, pp. 4736-38. The composite alloys, described in the references above, are generally deposited on a chromium or a chromium alloy under-layer with a body-centered-cubic (bcc) crystal lattice structure. Additionally, the chromium or the chromium alloy under-layer usually has a substantial [100] or [110] crystallographic orientation normal to the plane of the deposition substrate. This [100] or [110] crystallographic orientation of the bcc chromium or chromium alloy is well known in the art to be a useful under-layer for subsequent deposition of cobalt based alloys.
Cobalt-chromium-platinum with additions of less than 7% boron (% here and throughout this text is understood to mean atomic percent) are particularly useful for longitudinal magnetic recording media. Addition of boron to cobalt-chromium-platinum composite alloys has been proposed to enhance the anisotropic growth on the chromium under-layer causing some grains to be oriented with the c-axis in the plane of the film. However, this has not been observed for all compositions of cobalt-chromium-platinum alloys. Achieving this preferred orientation is important for these materials to be used in high density longitudinal magnetic media because the c-axis is the easy direction to magnetize and, therefore, it is preferably to have grains with their c-axes parallel to the plane (in-plane) of the longitudinal media for efficient write and read operations.
The addition of boron (less than 7 atomic %) to the composite alloy cobalt-chromium-platinum also increases the coercivity H.sub.c and the signal to noise ratio of the magnetic media as recognized by Doerner et al. in U.S. Pat. No. 5,523,173 where additions of 2-10 percent of B have been examined; M. Oka et al., U.S. Pat. No. 5,494,722. Further studies of this alloy have also been published by L. W. Song et al. in "Magnetic Properties and Recording Performance of Multi-layer Films of CoCrTa, CoCrPtTa, and CoCrPtTa with CoCrPtB", IEEE Transactions on Magnetics, Vol. 30, No. 6, November 1994, pp. 4011-13; P. Glijer et al. in "Magnetic Force Microscopy (MFM) Studies of Micromagnetic Structures of High Coercivity CoCrPt/Cr and CoCrPtB/Cr Thin Films", IEEE Transactions on Magnetics, Vol. 31, No. 6, November 1995, pp. 2842-2844.
Prior art teaches the advantages of cobalt-chromium-platinum-boron composite alloys containing less than 10% boron for magnetic structures deposited on bcc chromium or chromium alloy under-layers with substantial [100] or [110] crystallographic orientation normal to the plane of the deposition substrate. The benefits include, anisotropic grain growth (for some composition alloys) with the c-axis along in plane, increases in coercivity, and improved signal-to-noise ratio. There is a continued need to discover new compositions of magnetic materials for high density longitudinal magnetic media that have smaller grain size for improved signal to noise ratios while maintaining the high coercivity values.